"Shawn O. Pearce" <spearce@xxxxxxxxxxx> writes:
> Johannes Schindelin <Johannes.Schindelin@xxxxxx> wrote:
>> On Tue, 10 Feb 2009, Junio C Hamano wrote:
>> >
>> > I could do a revert on 'master' if it is really needed, but I found that
>> > the above reasoning is a bit troublesome. The thing is, if a tree to hold
>> > the notes would be huge to be unmanageable, then it would still be huge to
>> > be unmanageable if you split it into 256 pieces.
>>
>> The thing is, a tree object of 17 megabyte is unmanagably large if you
>> have to read it whenever you access even a single node. Having 256 trees
>> instead, each of which is about 68 kilobyte is much nicer.
>
> See my other email on this thread; we'd probably need to unpack
> all 256 subtrees *anyway* due to the distribution of SHA-1 names
> for commits.
I wonder if we can solve this by introducing a local cache that is a flat
file that looks like:
magic number for /usr/bin/file
tree object SHA-1 the file caches
Number of entries in this file
256 fan-out offsets into this file
N entries of <SHA-1, SHA-1>, sorted
Checksum of the file itself
and use it when availble (otherwise optionally create it upon the first
lookup). The file can be used by mmaping it and then doing a newton
raphson or binary search similar to the way patch-ids.c does.
The top-level API for such a hash-map would perhaps look like:
/*
* take the object name a tree object that is a hash map,
* return an opaque struct.
*/
struct hashmap *hashmap_open(const unsigned char *);
/*
* find the value given the key and return 0, or return negative
* if not found.
*/
int hashmap_lookup(struct hashmap *map, const unsigned char *key,
unsigned char *val);
/* discard the thing */
void hashmap_close(struct hashmap *map);
We should be able to use these in "git log" and friends where Dscho added
the hook in his git-notes topic.
I am hoping that I could eventually rewrite rerere to use something like
this, so that rerere database can be shared, just like the way notes can
be shared, across repositories.
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